Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Resistive switching memory elements typically include multiple metal oxide and nitride films between two electrodes as a resistive switching layer. The films are typically deposited as a stack of films, and are sometimes deposited using atomic layer deposition (ALD) processes. These multiple metal oxide and nitride films exhibit bistability, and can be placed in the high resistance state or low resistance state by applying the suitable voltages or currents.
During deposition of the metal oxide film on the bottom electrode, the bottom electrode can become oxidized. In particular, oxidation typically occurs during the oxidizer pulse steps of the ALD process. This oxidation of the bottom electrode can affect the electrical performance of the device, and, in particular, it can alter the switching properties of the device. This can affect the required currents and voltages necessary to reliably set, reset and/or determine the desired “on” and “off” states of the device, increase the overall power consumption of the memory chip, increase resistive heating of the device and increase cross-talk between adjacent devices.
Prior art techniques for solving the oxidation problem have involved surface pre-treatments of the bottom electrode prior to the ALD metal oxide deposition to prevent oxidation of the bottom electrode before and during the ALD deposition. However, these pre-treatment techniques need to be done ex-situ (out of the deposition chamber), which is disadvantageous because oxide can re-grow from exposure to air, prior to ALD deposition. Further developments and improvements are needed.